Many types of input devices may be used to provide input to computing devices, such as buttons or keys, mice, trackballs, joysticks, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation. Typically touch screens can include a touch sensor panel, which may be a clear panel with a touch-sensitive surface, and a display device that can be positioned behind the panel so that the touch-sensitive surface substantially covers the viewable area of the display device. Touch screens allow a user to provide various types of input to the computing device by touching the touch sensor panel using a finger, stylus, or other object at a location dictated by a user interface. In general, touch screens can receive a touch event and a position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Some capacitive-sense touch sensors can be formed from a matrix of row and column traces, capacitive sense elements or nodes present where the rows and columns cross over each other while being separated by a dielectric material. Each row can be driven by a stimulation signal, and touch locations can be identified through changes in the stimulation signal. Typically, a touch location is sensed based on an interference of the stimulation signal, such that a touch location may correspond to a location where the stimulation signal is the weakest.
In some instances it may be desirable for input devices, such as styli, to be able to transfer data, in addition to the touch location data, to the touch screen. For example, a stylus may have a finer tip than a user's finger and may be better able to transmit fine characters or symbols (such as those used in handwriting) better than a user's fingers. Additionally, using different input devices with a touch screen may allow an enhanced user experience, as data from the input devices (such as force, angle or tilt, or the like) may be communicated to the touch screen, which may use that data to change an output of the display or the like. However, in many instances, data communication between input devices and touch screens may require each of the input devices to be a slave to the touch screen, which may require a significant amount of power as the input devices may have to continuously listen to signals from the touch screen in order to start transmission in the correct window of time, to avoid colliding or otherwise interfering with the touch screen scan for touch inputs, such as from a user's fingers.